1. Field of the Invention
The present invention relates to a method for heating a glass body, which is particularly useful for fire polishing or cleaning, or heating and stretching a glass preform for an optical fiber.
2. Description of the Related Art
In general, a quartz base optical fiber is produced by forming a glass preform having an analogous cross sectional structure as that of an optical fiber to be fabricated and melting and stretching (spinning) the glass preform to fabricate the optical fiber.
To produce the glass preform, various methods have been practically used. In any method, the final glass preform from which the optical fiber can be fabricated is obtained after the glass preform or its precursor is thermally treated with an external heat source. Some of the examples of such thermal treatment are found in a VAD method, namely "fire polishing" by which a peripheral surface of the glass preform is smoothened with a flame just before spinning, and "stretching" of a transparent glass preform to a desired diameter.
For example, the fire polishing is carried out as shown in FIG. 1. That is, a glass preform 10 as a glass body is supported with a pair of rotatable chucks 11 and 12 and then heated with a burner 13 as a heat source. Since the burner 13 can heat a part of the glass preform 10 at one time, it is moved in a direction L in parallel with a longitudinal direction of the glass preform 10 so as to heat desired parts of the glass preform.
For example, the stretching of the glass preform is carried out as shown in FIG. 2. That is, the glass preform 10 is supported with a pair of chucks 11' and 12', one of which (the chuck 12' in FIG. 2) rotates the glass preform 10 as indicated by the arrow N and pulls it in the longitudinal direction M. Simultaneously, a burner 13' heats and softens a part of the glass preform 10 and moves in a direction L along the longitudinal direction of the glass preform as the glass preform is melted and stretched. The glass preform is stretched to a desired diameter by controlling the moving speeds of the chuck 12' and the heating rate with the burner 13'.
A flame generated with a conventionally used burner such as an oxyhydrogen burner, a natural gas burner and/or a petroleum gas burner can be used as the heating source. In addition, an electric furnace having a ring heater or a high-frequency induction furnace can be used as a heat source.
In the stretching of the glass preform, since the glass preform should be heated to a softening point, namely 1200.degree. C. or higher to its center part, the surface temperate reaches about 1500.degree. to 1600.degree. C.
In the case of fire polishing, the surface of the glass preform should be heated to about 1500.degree. to 1600.degree. C. to achieve sufficient surface smoothness.
To heat the surface of the glass preform to about 1500.degree. to 1600.degree. C. in the fire polishing and stretching of the glass preform, there arise the following problems:
1. Since the surface of the glass preform is heated, components of glass liberate from the surface and form fine particles of glass which float in an atmosphere near a heating part. The floating glass particles redeposit on a non-heated surface of the glass preform and contaminate the glass preform.
2. To remove the redeposited glass particles, only the redeposited glass particles are to be evaporated and the surface of the glass preform should be reheated at a temperature at which new glass particles are not formed. However, it is very difficult to evaporate all the redeposited glass particles by reheating, so that a part of the redeposited glass particles are melted and integrated with the glass preform to contaminate the surface of the glass preform.
In addition, when the glass preform has a large diameter, the surface of the glass preform is heated but the center part of the glass preform is not heated by the above reheating. Therefore, only the surface of the glass preform shrinks during cooling and residual stress remains on the surface of the glass preform and the produced glass preform tends to be easily broken.